v.parser: add check for existing type on sum type declaration (#11054)

vlib/v/parser/parser.v

@@ -3123,6 +3123,7 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 	p.check(.key_type)
 	end_pos := p.tok.position()
 	decl_pos := start_pos.extend(end_pos)
+	name_pos := p.tok.position()
 	name := p.check_name()
 	if name.len == 1 && name[0].is_capital() {
 		p.error_with_pos('single letter capital names are reserved for generic template types.',
@@ -3197,6 +3198,11 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 			}
 			is_public: is_pub
 		})
+		if typ == -1 {
+			p.error_with_pos('cannot register sum type `$name`, another type with this name exists',
+				name_pos)
+			return ast.SumTypeDecl{}
+		}
 		comments = p.eat_comments(same_line: true)
 		return ast.SumTypeDecl{
 			name: name
@@ -3233,7 +3239,7 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 	type_end_pos := p.prev_tok.position()
 	if idx == -1 {
 		p.error_with_pos('cannot register alias `$name`, another type with this name exists',
-			decl_pos.extend(type_alias_pos))
+			name_pos)
 		return ast.AliasTypeDecl{}
 	}
 	if idx == pidx {

vlib/v/parser/tests/sum_type_exists_err.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/sum_type_exists_err.vv:1:6: error: cannot register sum type `Option`, another type with this name exists
+    1 | type Option = string | int
+      |      ~~~~~~

vlib/v/parser/tests/sum_type_exists_err.vv

@@ -0,0 +1 @@
+type Option = string | int

vlib/v/parser/tests/type_alias_existing_type_err.out

@@ -1,7 +1,7 @@
-vlib/v/parser/tests/type_alias_existing_type_err.vv:3:1: error: cannot register alias `Foo`, another type with this name exists
+vlib/v/parser/tests/type_alias_existing_type_err.vv:3:6: error: cannot register alias `Foo`, another type with this name exists
     1 | struct Foo{}
     2 | 
     3 | type Foo = Foo
-      | ~~~~~~~~~~~~~~
+      |      ~~~
     4 | 
     5 | fn main() {